The present invention relates to hydrostaticly driven vehicles and is particularly directed to a linkage for controlling two independent hydraulic drive units.
Hydrostatic drive systems are in common use on all sorts of vehicles, as shown by the following U.S. Pat. Nos. 3,810,571 (loader), 3,581,497 (mower-conditioner), and 3,880,251 (trencher). Many of these vehicles have two speed ranges, a high range for road travel and a low range for working conditions. A common means used to provide these vehicles with both high and low speed ranges is a two speed motor or motors, typically being a variable displacement motor of the conventional axial-piston type. This type of motor has an internal swash plate whose angle may be varied to change the stroke of internal pistons and thereby vary the outut speed of the motor. To establish desired high and low speed ranges for the motor, some type of stops, either internal or external to the motor, are provided to define predetermined displaced positions for the swash plate. Thus, when the swash plate is held against one of the stops, the motor would be in the high speed range and when the swash plate is held against the other stop, the motor is in the low range. The swash plate is moved between the high and low speed stops by a control arm connected to a pintle shaft about which the swash plate pivots.
The swash plate is subjected to extremely high pressures exerted thereon by the reciprocating action of the pistons. Consequently, if the swash plate is not held firmly against the stop, the swash plate will vibrate and thus cause wear which results in costly repairs. This vibration also creates a disturbing noise sometimes referred to as chatter.
The above-described problem predominately occurs when two or more independent motors are to be controlled simultaneously. Because of manufacturing tolerances of the motors and associated control arms, the control arm of one motor may have to be pivotally moved through a different angular displacement than that of the control arm of another motor in order for the swash plate of each motor to reach its respective stop. Up until now, no satisfactory linkage has been provided which enables the control arm and therewith the swash plates of two or more independent motors to be simultaneously moved and held firmly against their respective stops.